Molybdenum occurs in nature most commonly as molybdenite (MoS.sub.2). While molybdenite may be the primary metal value of an ore body, such as that at Climax, Colo., it is often found as a secondary metal value in a copper ore body, such as that at Bingham Canyon, Utah.
Copper ores only rarely contain sufficient copper to permit direct smelting, and many ores contain less than 1% copper. The copper content of these thin ores must be significantly increased before these materials are worthy to serve as a smelter feed and to this end, these thin ores are subjected to concentration. In this process, the ores are crushed and ground to expose their copper mineralization, and then floated in a series of flotation cells in which the copper minerals are recovered as a froth concentrate and the noncopper-bearing minerals, e.g. silicates and carbonates generally known as gangue, are recovered as tailings.
Many copper flotation facilities comprise three banks of flotation cells, i.e. rougher cells, cleaner cells and scavenger cells. The ore slurry produced during the crushing and grinding of the ore is feed for the rougher cells in which most of the copper mineralization is floated. The froth concentrate from the rougher cells is collected and transferred to the cleaner cells in which much of the remaining gangue is rejected and recycled, while the clean copper concentrate is dewatered and readied for use as a smelter feed. The material that does not float in the rougher cells is transferred to the scavenger cells in which additional copper is recovered. The froth from the scavenger cells is processed to separate gangue from copper mineralization, and the mineral values are returned to the rougher cells.
If molybdenite is present in a copper ore body, then it will usually float with the copper mineralization. As such, the copper concentrate from the cleaner cells is usually processed in a separate flotation circuit to remove the molybdenite before the copper concentrate is readied as a feed to the smelter. The molybdenite is recovered as a molybdenite concentrate, e.g. typically in excess of 90% MoS.sub.2 with the remainder mostly silicates and carbonates and various, usually nominal, amounts of other metals such as copper, gold, arsenic, etc. The molybdenite concentrate is then processed to produce molybdenum trioxide which is used primarily as an alloying agent in the production of specialty steels.
If the copper ore body contains nonmetal-bearing, naturally floatable silicate gangue minerals, such as talc and/or sericite, then these minerals will form slimes (because of their soft character), and these slimes tend to follow the copper mineralization during flotation. These slimes are difficult to separate from the molybdenum values and when such a separation is attempted (e.g. by flotation or cycloning), it usually results in the loss of a relatively large amount of the molybdenum values.
Various methods exist or have been proposed for producing molybdenum trioxide from molybdenite concentrate. The dominant technology is roasting in which the concentrate is heated in the presence of excess air to form molybdenum trioxide and sulfur dioxide as a gaseous by-product. While proven, this technology is environmentally difficult and produces an off gas with a low concentration of sulfur dioxide which requires upgrading before it is an economically attractive feed to an acid plant. Additionally, roasting is, as a practical matter, limited to molybdenum concentrates that contain less than 5 wt % copper and less than a total of 10 wt % combined naturally floatable gangue minerals such as talc and sericite. The presence of these substances results in the formation of a sticky material in the roaster that adheres to the rabble arms of conventional multihearth roasters, and interferes with the rejection of fixed sulfur.
One variation on roasting is combining it with sublimation as described in such patents as U.S. Pat. Nos. 3,848,050, 3,910,767, 4,555,387, 4,551,313 and 4,551,312. This process has the merits of producing an off gas relatively rich in sulfur dioxide but remains unproven (i.e. it is yet to be commercialized) and suffers from relatively high losses of molybdenum to byproduct slag produced in the process.
Another variation on roasting is combining it with either a pre- or post treatment step in which the concentrate is contacted with a suitable reagent, e.g. ferric chloride, hydrochloric acid, sodium cyanide, ferric sulfate, sulfuric acid, etc., to remove deleterious base metal impurities such as copper. While generally effective, these variations require, by definition, an extra process step, and the various treatment reagents all have their own undesirable baggage, e.g. cyanide compounds are environmentally disfavored; chloride ion, ferric sulfate and sulfuric acid are corrosive, etc.
Another class of processes for the production of molybdenum trioxide from molybdenite concentrate are hydrometallurgical in nature. In these processes, the concentrate is leached with one of various reagents, e.g. hypochlorite ion. While these processes avoid the production of an off gas, all suffer other disabilities, e.g. hypochlorite is a relatively expensive reagent, and most remain unproven.
One hydrometallurgical process with promising economics and compatibility with the environment is pressure oxidation. In this process, the molybdenite concentrate is slurried with water, and then it is fed to an autoclave in which it is contacted with oxygen under pressure. The process can be conducted either continuously or on a batch basis. Insoluble molybdenum trioxide (MoO.sub.3) is recovered by filtration. Several varients of this process are described generally in German patent documents DE3,128,921 and DE2,830,394 as well as U.S. Pat. Nos. 3,656,888; 4,379,127, and 4,512,958.
While all of the known processes for producing molybdenum trioxide from molybdenite concentrate are effective to one degree or another, the mining industry holds a continuing interest for a process that is not only economically efficient, but also has a low environmental impact. In addition, the industry has a continuing interest in developing the ability to process those grades of molybdenite concentrates that contain relatively high levels of impurities such as copper and naturally floatable gangue, e.g. talc and sericite, which are presently difficult to roast to yield molybdenum trioxide of at least technical grade.